Download Air Masses and Fronts

Air Masses and Fronts
Chapter 11
Air Masses
• Extremely large body of air whose
temperature and humidity are similar
horizontally and vertically.
• Source Regions: area where air mass
originates, usually flat and uniform
composition with light surface winds
Here, a large, extremely cold winter air mass is dominating the weather over much of
the United States. At almost all cities, the air is cold and dry. Upper number is air
temperature (°F); bottom number is dew point (°F).
• Classification
– Classification based upon temperature and
humidity
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P = polar
T = tropical
A = Arctic
m = maritime
c = continental
• North America cP and cA
– Source region: N. Canada, Alaska
– Dry, cold, stable (A more extreme)
Average upper-level wind flow (heavy arrows) and surface position of anticyclones (H)
associated with two extremely cold outbreaks of arctic air during December. Numbers
on the map represent minimum temperatures (°F) measured during each cold snap.
Stepped Art
Typical vertical
temperature profile
over land for a
summer and a
winter cP air mass.
Visible satellite image showing the modification of cold continental polar air as it
moves over the warmer Gulf of Mexico and the Atlantic Ocean.
• North American mP
– Source region: North Pacific, North Atlantic
– Cool, moist, unstable
Clouds and air flow aloft
(large blue arrow)
associated with maritime
polar air moving into
California. The large L
shows the position of an
upper-level low. Regions
experiencing precipitation
are also shown. The
small, white clouds over
the open ocean are
cumulus clouds forming in
the conditionally unstable
air mass.
Stepped Art
Winter and early spring surface weather pattern that usually prevails during the invasion
of cold, moist mP air into the mid-Atlantic and New England states. (Green-shaded area
represents light rain and drizzle; pink-shaded region represents freezing rain and sleet;
white-shaded area is experiencing snow.)
• North American mT
– Source region: Gulf of Mexico, Caribbean, SE Pacific
– Wet, warm, unstable
– Pineapple Express and Bermuda High
An infrared satellite image
that shows maritime
tropical air (heavy yellow
arrow) moving into
northern California on
January 1, 1997. The
warm, humid airflow
(sometimes called “the
pineapple express”)
produced heavy rain and
extensive flooding in
northern and central
Californ
Weather conditions during an unseasonably hot spell in the eastern portion of the United
States that occurred between the 15th and 20th of April, 1976. The surface low-pressure
area and fronts are shown for April 17. Numbers to the east of the surface low (in red)
are maximum temperatures recorded during the hot spell, while those to the west of the
low (in blue) are minimum temperatures reached during the same time period. The
heavy arrow is the average upper-level flow during the period. The purple L and H show
average positions of the upper-level trough and ridge.
• North American cT
– Source Region: SW US, Mexican Plateau
– Hot, dry, stable
From July 14 through July 22,
2005, continental tropical air
covered a large area of the
southwestern United States.
Numbers on the map represent
maximum temperatures (°F)
during this period. The large H
with the isobar shows the
upper-level position of the
subtropical high. Sinking air
associated with the high
contributed to the hot weather.
Winds aloft were weak, with the
main flow over central Canada
Fronts
•
•
Transition zone between two air masses of
different densities
Identification on Charts
1.
2.
3.
4.
5.
Sharp temperature change
Sharp change in dew point
Shift in wind direction
Sharp pressure change
Clouds and precipitation
Symbols:
• Stationary Front
– Front with no movement
– Alternating red and blue line with blue
triangles and red semi-circles
– Winds parallel but opposite direction
– Variable weather
• Cold Front
– Cold, dry stable air replaces warm, moist
unstable air
– Blue line with blue triangles
– Clouds of vertical development
– Thunderstorms, squall lines
A closer look at the surface
weather associated with the cold
front situated in the southern
United States i (Gray lines are
isobars. Green-shaded area
represents rain; white-shaded
area represents snow.)
A vertical view of the weather across the
cold front along the line X–X’.
The infrared satellite image (a) shows a weakening cold front over land on Tuesday
morning, November 21, intensifying into (b) a vigorous front over warm Gulf Stream
water on Wednesday morning, November 22.
A “back door” cold front moving into New England during the spring. Notice that,
behind the front, the weather is cold and damp with drizzle, while to the south, ahead
of the front, the weather is partly cloudy and warm.
• Warm Front
– Warm, moist unstable air overrides cold, dry
stable air
– Red line with red semi-circles
– Horizontal cloud development with steady
rain
Surface weather associated with a typical warm front. A vertical view along the dashed line P-P’
is shown in Fig. 11.21. (Green-shaded area represents rain; pink-shaded area represents
freezing rain and sleet; white-shaded area represents snow.)
Vertical view of clouds, precipitation, and winds across the warm front i along the line P–
P’.
Cold Fronts and Warm Fronts
A dryline represents a narrow boundary where there is a steep horizontal change
in moisture as indicated by a rapid change in dew-point temperature. Here, a dryline
moving across Texas and Oklahoma separates warm, moist air from warm, dry air
during an afternoon in May.
• Occluded Front
– Cold front catches up to and over takes a
warm front
– Purple line with purple triangles and semicircles
– 2 types: Cold occlusion, warm occlusion
The formation of a cold-
occluded front.
The faster-moving cold
front (a) catches up to the
slower-moving warm front
(b) and forces it to rise off
the ground(c).
(Green-shaded area in
(d) represents
precipitation.)
The formation of a warm-type
occluded front. The fastermoving cold front in
(a) overtakes the slowermoving warm front in
(b). The lighter air behind the
cold front rises up and over the
denser air ahead of the warm
front.
Diagram (c) shows a surface
map of the situation.
A visible satellite image
showing a mid-latitude
cyclonic storm with its
weather fronts over the
Atlantic Ocean during
March, 2005. Superimposed
on the photo is the position
of the surface cold front,
warm front, and occluded
front. Precipitation symbols
indicate where precipitation
is reaching the surface
• Upper-Air Fronts
– Front aloft
– Tropopause dips downward and folds under
the Polar jet
– Impacts surface weather